Positron annihilation techniques have been employed to investigate the formation of vacancy type of compensation defects in undoped LEC-grown InP. N-type InP becomes p-type semiconducting by short time annealing at 700 degreesC, and then turns to be n-type again after further annealing but with a much higher resistivity. Positron lifetime measurements show that the positron average lifetime tau (av) increases to a high value of 247ps for the first n-type to p-type conversion and decreases to 240ps for the following p-type to n-type conversion. tau (av) increases slightly and saturates at 242ps upon further annealing. The results of positron annihilation Doppler-broadening measurements are consistent with the positron lifetime measurements. The correlation between the characteristics of positron annihilation and the conversions of conduction type indicates that the formation of vacancy type defects and the progressive variation of their concentrations during annealing are critical to the electrical properties of the bulk InP material.